/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.iotdb.db.query.reader.universal;

import org.apache.iotdb.db.utils.TimeValuePairUtils;
import org.apache.iotdb.tsfile.file.metadata.enums.TSDataType;
import org.apache.iotdb.tsfile.read.TimeValuePair;

import java.io.IOException;

/**
 * CachedPriorityMergeReader use a cache to reduce unnecessary heap updates and increase locality.
 */
public class CachedPriorityMergeReader extends PriorityMergeReader {

    private static final int CACHE_SIZE = 100;

    private TimeValuePair[] timeValuePairCache = new TimeValuePair[CACHE_SIZE];
    private int cacheLimit = 0;
    private int cacheIdx = 0;

    private Long lastTimestamp = null;

    public CachedPriorityMergeReader(TSDataType dataType) {
        for (int i = 0; i < CACHE_SIZE; i++) {
            timeValuePairCache[i] = TimeValuePairUtils.getEmptyTimeValuePair(dataType);
        }
    }

    @Override
    public boolean hasNextTimeValuePair() {
        return cacheIdx < cacheLimit || !heap.isEmpty();
    }

    private void fetch() throws IOException {
        cacheLimit = 0;
        cacheIdx = 0;
        while (!heap.isEmpty() && cacheLimit < CACHE_SIZE) {
            Element top = heap.peek();
            if (lastTimestamp == null || top.currTime() != lastTimestamp) {
                TimeValuePairUtils.setTimeValuePair(
                        top.getTimeValuePair(), timeValuePairCache[cacheLimit++]);
                lastTimestamp = top.currTime();
            }
            // remove duplicates
            while (heap.peek() != null && heap.peek().currTime() == lastTimestamp) {
                top = heap.poll();
                if (top.hasNext()) {
                    top.next();
                    heap.add(top);
                } else {
                    top.close();
                }
            }
        }
    }

    @Override
    public TimeValuePair nextTimeValuePair() throws IOException {
        TimeValuePair ret;
        if (cacheIdx < cacheLimit) {
            ret = timeValuePairCache[cacheIdx++];
        } else {
            fetch();
            ret = timeValuePairCache[cacheIdx++];
        }
        return ret;
    }

    @Override
    public TimeValuePair currentTimeValuePair() throws IOException {
        if (0 <= cacheIdx && cacheIdx < cacheLimit) {
            return timeValuePairCache[cacheIdx];
        } else {
            fetch();
            return timeValuePairCache[cacheIdx];
        }
    }
}
